It may be the fine print on the specific model. I'm using the 8.5" reflector dome with rheostat/dimmer from Flukers with a 100w CHE and having zero issues. Using this one, which includes text specifically referencing CHE bulb usage.

Dad's probably worried that the wire frame will cause heat loss instead of reflecting the heat into the enclosure. With my current dome, the bulb sits an inch or two above the screen at least, though this lamp came with an arm to use if necessary.